Since it was invented in 1958, IC has been scaled to improve the performance. However, after the turn of the century, scaling resulted in short-channel effect and memory wall, which requires other approach to improve the IC performance more effectively.
3D IC offers a reasonable route to further improve IC performance. It improves IC performance by increasing device density, reducing the interconnect delay, and breaking memory wall with the application of 3D stacking using through silicon via (TSV). 3D IC also makes one chip package have more functional diversification than those enhanced only by shrinking the size of the features. The main advantages of 3D IC are the smaller form factor, low energy consumption, high speed, and functional diversification. It is predicted that 3D IC will be an enabler for improvement of data center performance and efficiency with positive consequences for global energy consumption and environment.
One of the biggest challenges in 3D IC stacking technology using TSV is thermal management owing to the high heat flux up to about 200 watts per square centimeter. Disclosure of this invention will provide a solution of 3D IC thermal management.
Data centers for internet and mobile devices are the most critical components in our information age. They serve industries, civil communications, military and defense applications, and transportations. Data centers consist of multiple computers usually called servers and switches. Both of them use many ICs. When a computer works, ICs will change status, or change the on-and-off status, which consumes electricity and generates significant heat. Even when computer system is at idle condition, it still consumes electricity due to the current leakage and circuit requirement.
Multiple servers are accommodated in a server rack at data center. Each computer consumes significant electricity. It is common for a server (computer) to consume over a hundred watts. In a server rack, i.e. a module of servers, there are multiple computers. Similarly, there are many server racks in a data center. Therefore, a data center consumes large amount of electricity and a large data center consumes the same amount of electricity as a small or medium size town. Among the contributions to the electricity consumption, most electricity is consumed by servers and their cooling systems. It is quite often that cooling system uses the same amount of electricity as the server computers. It is estimated that the date centers consume about two percent of total electricity generated worldwide.
Power usage effectiveness (PUE) is usually used to measure the efficiency of a data center. It is defined as a ratio of total energy used by facility to that used by information technology (IT) equipment. An ideal PUE is 1.0, but average PUE worldwide now is about 2.0 although some data center claims their PUE is significantly below 2.0. The average PUE value of 2.0 indicates the necessity to improve the data center cooling effectiveness. One approach to improve the cooling efficiency is to use water cooling to replace current air cooling. In the past, water cooling was used for large scale computers, but did not obtain large scale application for personal computers or servers in data center because of its limitation by the shape of heat-generating components and thus the complexity.
As the dimensions of integrated circuit components decrease, more components are compacted in a given area of a semiconductor integrated circuit (for example of 3D IC). Accordingly, more transistors are held on a given area and thus more heat is generated in the same area. In order to keep the IC temperature in allowed range for proper performance, heat generated has to be transferred out of integrated circuit effectively and economically. With the internet getting popular, more and more servers are installed and in service to support the internet function. The trend of applications of more mobile devices and cloudy technology will drive more electricity consumption at data centers in the future.
Current servers are located in an air-conditioner-regulated environment, usually in a specially designed building. The heat generated by microprocessors, memory chips, and power supply chips is released locally, which is like a large heater in a room cooled by air conditioner. Due to the low efficiency of air conditioner, the cooling system uses lots of electricity, occupies large footprints, and causes high costs.
Accordingly, it is very significant to provide an effective method to reduce cooling power and improve cooling efficiency for computer system, especially for the system with large number of computers such as data center. Cooling technology now becomes an enabler to improve data center efficiency.
Improving cooling system in data center not only saves energy consumption, but also benefits ecological and environmental systems. A few percent reduction of electricity consumption in data center cooling system will significantly decrease the emission of carbon dioxide amount, which equivalents to shut down multiple coal power plants with environmental benefit in addition to the cost reduction.
The heat generated in electronic devices in a data center has to be transferred outside the accommodating construction and dissipated to environment, which consumes tremendous electricity. In order to prevent the overheat of ICs, the surface of the ICs should be kept not very high, which means the temperature difference between high temperature source (IC surface) and low temperature environment will be significant low, resulting in the challenge in engineering realization and high costs in cooling system.
Traditionally, heat-generating components in computers are cooled by cold air supplied by air-conditioners. The air in server's building exchanges and dissipates heat on chiller's cold surface. By applying work, air conditioners transfer heat from a cold surface to a hot surface, and thus heat is dissipated to air outside the building by heat exchanging. This cooling method is accompanied with uses of lots of compressors and fans, and thus consumes significant electricity because of the low efficiency and high costs for air conditioning system.
In order to lower the cost of using air conditioner, cold air is used to directly cool the heat generating components in winter at north areas. However, the air humanity has to be controlled well and the application is also limited by weather and season.
Similarly, lots of power is used by fans in the server rack to dissipate heat from component surface to air by blowing air through the server rack, which also consumes significant energy, makes noise, and has low efficiency.
In order to overcome low efficient challenge in air cooling problems, water is used for cooling the heat-generating components. Current heat-generating components are mainly microprocessor unit (MPU), dynamic random-access memory (DRAM), and power chips. Microprocessor has a flat shape and it is relatively easy to use liquid cooling on a flat surface. However, it is difficult to use liquid cooling on DRAM dual in-line memory module (DIMM) due to the irregular shape although some attempts were tried.
In order to overcome the intrinsic problem mentioned above, liquid cooling was used by circulating liquid coolant on the surface of ICs to improve the efficiency. However, this method has to use chillers to cool the liquid, resulting in a low cooling efficiency.
In order to use natural water body for data center cooling, air cooling of server rack was combined with heat dissipation to large natural water bodies such as ocean, river, and lake. This approach may be the lowest data center operating cost and has the best potential for future application. However, there are lots of challenges for the realization of this method. Therefore, some novel method is disclosed in this invention for improving server cooling and data center efficiency.